(a) Field of the Invention
The present invention relates to a zoom lens system for a camera and, more particularly, to a zoom lens system having a first lens group of four lenses and a second lens group of three lenses, one of which is an aspherical lens.
(b) Description of the Related Art
Generally, zoom lens systems for compact cameras are divided into two types: those having two lens groups and those having three lens groups. The two lens group type is compact and lightweight, but does not provide a zooming ratio greater than 2.0 without using an aspherical lens.
Using an aspherical lens, it is possible to obtain a zooming ratio as great as 2.4. In order to obtain a zooming ratio higher than 2.4, it is necessary to use more lenses or reduce an effective diameter of the lenses in order to obtain high quality images. If a zoom lens system has more lenses, the total length of the system increases so that a telephoto ratio of the system also increases to more than 1.0. It becomes difficult to achieve a compact zoom lens system.
Because a two lens group type zoom lens system for a compact camera generally has a focal length of 35-39 mm, a viewing angle becomes about 63.4-59.0 degrees for a general Leica size image plane (24.times.36 mm). As a viewing angle increases, it becomes more difficult to correct aberrations over the entire zoom lens system.
In order to overcome the above drawbacks, zoom lens systems having two lens groups are disclosed in Japanese Patent Application Laid-open Publications Nos. Hei 3-260610, Hei 4-93810, Hei 7-234363 and Sho 52-52111. The zoom lens systems disclosed in the above prior art documents comprise a first lens group of a positive refractive power and a second lens group of a negative refractive power. The focal length of the system depends on the distance between the first lens group and the second lens group. Each of these prior art zoom lens systems has a zooming ratio less than 2.4 and uses one or more aspherical lenses.
The zoom lens system of Hei 7-234363 has an ultra wide viewing angle of more than 70 degrees while the other zoom lens systems have viewing angles less than 65 degrees.
These prior art zoom lens systems also have telephoto ratios greater than 1.0.
Therefore, a zoom lens system with a high zooming ratio and a wide viewing angle has more lenses than a system with a zooming ratio less than 2.4 and a viewing angle less than 70 degrees. As a result, it is difficult to reduce the size of the system, making it difficult to obtain a telephoto ratio below 1.0.